Homeowners and owners of small businesses are increasingly concerned about protecting buildings from intruders. Higher labor costs make it increasingly difficult to provide guards and patrol services, and so reliance on protective devices such as burglar alarms is necessary. Cost and reliability are major factors in selecting this type of equipment, as well as the desire to make the equipment as unobtrusive as possible. Windows may be protected by conductors taped to the glass, but devices of this type are rather unattractive, and the cost of installing may be high in materials and labor. Object detection schemes have been developed which make use of sound detection, light beam interruption, electromagnetic field interruption, etc., but these are of high cost and need to be carefully tuned and adjusted to avoid false alarms yet permit detection with reasonable reliability. Accordingly, a primary feature of this invention is that of providing a protective or intrusion detector technique for buildings or the like which is effective and reliable, yet of low cost and easily installed and operated.